Cu/Al Through Au Diffusion Characterized by KPFM

Abstract

The nanometer scale characterization technique of Frequency Modulated Kelvin Probe Force Microscopy (FM-KPFM) will be used to assess a diffusion study on thin metal films that undergo accelerated aging. The KPFM technique provides a relatively easy, non-destructive methodology that does not require high-vacuum facilities to obtain nanometer-scale spatial resolution of surface chemistry changes. The KPFM technique will be exercised in an effort to explore its capacity to map surface potential contrast caused by diffusion in a manner that allows for a qualitative assessment of diffusion of Cu or Al through Au. Supporting data will be obtained from traditional techniques: AES, XPS and UPS. An aging study was conducted on thin metal test specimens comprised of 500nm Cu or Al then 500nm Au on Si. The accelerated aging process was performed under inert conditions at aging temperatures of 100°C for Cu/Au film stack and 175°C for Al/Au film stack at aging times of 8 hours, 24 hours, 96 hours (4 days), and 216 hours (9 days).A calibration method was developed using Au, Al and Cu standards to establish precision and repeatability of the KPFM technique. The average Contact Potential Difference (CPD)s and standard deviations for each metal were found and summarized. Averages from surface roughness of the AFM topography images and roughness analysis of KPFM potential images which yield an average CPD of each area of unaged vs aged coupon surfaces were compared and show trends that indicate surface chemistry.